【作者】 张雪林；

【导师】 姜兆华；

【作者基本信息】 哈尔滨工业大学 ， 化学工艺， 2009， 博士

【摘要】 等离子体电解氧化(PEO)陶瓷膜层的生长过程存在着电阻以及电容等阻抗特性的变化,这种阻抗特性的变化与陶瓷膜的结构以及PEO过程中放电方式的变化密切相关,因此通过研究陶瓷膜层生长过程中阻抗特性的变化,特别是从微弧到弧放电阶段,就可以实现对PEO过程的各个阶段的监控,这对于制备和获得高质量高性能的膜层具有重要的指导意义。本文采用电化学阻抗(EIS)技术与扫描电镜等形貌观察相结合,对钛合金表面PEO陶瓷膜在各个生长阶段的阻抗特性进行了研究,最后通过对PEO反应过程中的电压电流响应的在线测量,获取反应过程中的等效电阻与等效电容等动态阻抗的相关信息并建立其与反应过程中各阶段的联系,通过动态阻抗特性的变化来实现对各个阶段的监控。同时本文还对陶瓷膜在不同溶液中进行了动电位极化测试,分别用于陶瓷膜的EIS拟合结果的验证以及陶瓷膜的耐蚀性能的研究。以TC4钛合金在3.5%NaCl溶液中的腐蚀行为为例,详细的分析了充电电流对极化曲线的形状以及动力学参数的影响。由于充电电流与扫描速率和扫描方向有关,对于Ti6Al4V与PEO陶瓷膜,可以分别在0.5mV/s和0.05mV/s的扫描速率下通过对正反向扫描过程中的外测电流取平均值的方法有效地减弱充电电流的干扰。通过对PEO陶瓷膜在铝酸钠工作液中直接进行电化学阻抗谱测试来研究膜层在生长过程中的阻抗特性的变化以及工艺参数的影响,结果表明:在火花阶段,随处理电压的升高,PEO陶瓷膜的致密程度增加,在工作液中的极化电阻值、电容值均增加,在微弧阶段,大火花放电对膜层具有一定的破坏作用,导致膜层的极化电阻值变小,但电容值明显增加。直流电源模式下,随电流密度的增加,陶瓷膜在工作液中的极化电阻值增加;火花阶段陶瓷膜疏松层的结构受电流密度的影响很小,陶瓷膜在工作液中的电容值也基本不变;微弧阶段陶瓷膜疏松层的结构受电流密度影响很大,电容值随电流密度的增加而增加。单向脉冲模式下很容易发生弧放电,严重的破坏了膜层的致密程度,使得陶瓷膜在工作液中的极化电阻值与电容值均减小;随电源峰值电流密度的增加、频率的提高以及占空比的减小,发生弧放电时的电压提高。单向脉冲模式下PEO反应过程电源波形的测试结果表明:在单个脉冲周期内,电压阶跃很快就能完成,但是电流随时间而成指数衰减至稳定的数值,电流衰减速度随处理电压的变化可以分为三个阶段,分别与PEO膜层生长过程中的火花阶段、微弧阶段以及弧放电阶段的电压范围相对应,据此可以对膜层的生长过程进行监测。对电流衰减曲线进行拟合,结果表明:在火花阶段,放电通道的底部非常接近或者直接与基体相接触,进入微弧阶段之后,放电通道的底部与基体之间被逐渐分隔开来;PEO反应过程中动态电阻的数值在发生弧放电之前随处理电压的升高而增加,弧放电的发生则使得动态电阻的数值减小,动态电容在火花阶段随处理电压的升高而减小,在微弧阶段则开始增加,进入弧放电之后又开始减小。本文还对Ti6Al4V经过PEO处理之后的耐蚀性能进行了研究,结果表明在3.5%的NaCl溶液中PEO陶瓷膜的腐蚀电流密度比基体合金降低了一个数量级。火花阶段的陶瓷膜的耐蚀性能随处理电压的提高而增强,但是发生大火花放点之后,陶瓷膜的耐蚀性能下降。对于单向脉冲模式下制备的陶瓷膜,随频率的增加、占空比的降低,陶瓷膜的耐蚀性能增加。电源参数对PEO陶瓷膜耐蚀性能的影响与陶瓷膜在工作液中EIS拟合结果的内层电阻的变化相一致。更多还原

【Abstract】 The impedance characteristic of the plasma electrolytic oxidation (PEO) coating depends strongly on the coating structure. By studying the impedance characteristic, the growth process of the PEO coating can be well understood. During the PEO process, the response of the power source is affected by the impedance characteristic of the breakdown coating. By observing the response of the power source, i.e., the wave shape of the current or voltage, the growth process of the PEO coating can be controlled, especially for the transfer of micro-arc stage to arc stage. Doing so is very helpful for the preparation of the PEO coating of high performance.Based on the above purpose, electrochemical impedance spectroscopy, combined by SEM, EDS, XRD analysis, are adopted to study the impedance characteristic of the PEO coating prepared at different stages. The response of the single-pulse power during the PEO progress is recorded by the oscillograph, and analyzing the waves of the current and voltage presents the impedance characteristic of the breakdown coating, which includes the active resistance during the breakdown, Ra and the capacitance of the part where the breakdown does not occur, C. Moreover, the potentiodynamic polarization is also performed in different solutions to testify the fitting results of EIS and to study the corrosion resistance of the coating.For the corrosion system of very low corrosion rate, the potentiodynamic polarization is more readily disturbed by the charging current. In this paper, the corrosion behavior of TC4 in 3.5% NaCl solution is taken as an example to illustrate how the potentiodynamic polarization curve and the obtained kinetics parameters are affected by the charging current. Based on the relation of the charging current with the scan direction, the disturbance of the charging current can be eliminated or reduced greatly by averaging the two current densities measured by positive and negative scan, respectively.The EIS spectra measured in the working solution can reflect well the growth character of the coating during the PEO process. In spark stage, the equivalent resistance and capacitance of the coating increase with the treatment voltage. In micro-arc stage, the equivalent resistance and capacitance decreases and increases, respectively. For the coating prepared by DC power, the outer and inner layers are nearly not affect by the current density in spark and micro-arc stage, respectively. For the coating prepared by single-pulse power source, the arc discharge is easily to happen, which causes great damage to the coating and makes the equivalent resistance and capacitance both decrease. With the increase of the frequency and the peak current density, the voltage where the arc discharge occurs increases, and the decrease of the duty ratio also makes the arc discharge voltage increase.During the PEO process, the real wave shape of the output of the power was very different from its theoretical shape. At the initial stage of the pulse, about within 1μs, the voltage and the current reached a maximal value at the same time, afterward, the voltage fall down to a stable value quickly, but the current decayed exponentially in a long period to the stable value, which was exactly like the behavior of the parallel component of a resistor and a capacitor. The variation of the decay rate of the current with the treatment voltage can be divided into three stages, which corresponded to the spark stage, micro-arc stage and arc stage of the PEO process. The fitting results of the current decay curve showed that at the early stage of the PEO process, the bottom of the discharge channel was very close or contacted directly with the substrate, but at the later stage of the PEO process, the discharge channel was separated from the substrate by coating. With the increase of the treatment voltage, the active resistance Ra increases before arc stage; the capacitance C decreases in spark stage, then increases in micro-arc stage, and it decreases again in arc stage.This paper also deals with the corrosion resistance of the PEO coating. After PEO treatment, the corrosion current density of TC4 decreases by more than one orders. The corrosion resistance of the PEO coating increases with the treatment voltage in spark stage, and gets worse when the PEO process comes into micro-arc stage. For the coating prepared by single-pulse power source, the increase of the frequency and decrease of the duty ratio can lead to the improvement of the corrosion resistance of the PEO coating. The influence of the technical parameters on the corrosion resistance is exactly in the same way as their influence on the resistance value of the inner layer in the equivalent circuit established to the fit the EIS spectra.更多还原